A conventional ICP-MS instrument is first described by referring to FIG. 1. This instrument comprises an ICP ion source 1 which is composed of a plasma torch 3 made of an electrical insulator such as quartz, a nebulizer 21 for atomizing liquid sample 6, and an argon gas source 22 for supplying argon gas to both torch 3 and nebulizer 21. An RF (radio frequency) coil 2 is wound around the torch 3. A sample bottle 5 holds the sample 6 and is connected with the nebulizer 21 via an intake pipe 7. The torch 3 with the RF coil 2 is surrounded by a grounded shield case (not shown) to prevent leaking of RF fields from the RF coil 2.
An interface 8 comprises a sampling cone 9 made of an electrical conductor, a first skimmer 10, and a second skimmer 11. A mass spectrometer 12 incorporates a mass analyzer 13 consisting either of a quadrupole mass spectrometer or of a double-focusing mass spectrometer having both an electric sector and a magnetic sector.
An oil diffusion pump 14 acts to maintain the inside of the mass spectrometer 12 as a high vacuum. A rotary oil-seal pump 15 evacuates a space S.sub.1 formed between the sampling cone 9 and the first skimmer 10 via an evacuation pipe 17. Similarly, an oil diffusion pump 16 evacuates a space S.sub.2 formed between the first skimmer 10 and the second skimmer 11 via an evacuation pipe 18.
Electrodes 19 converge ions to direct them into the mass analyzer 13. Accelerating electrodes 20 are mounted between the second skimmer 11 and the electrodes 19. Since the kinetic energies of ions to be analyzed are restricted within a range from 0 to 20 eV in quadrupole mass spectrometry, the sampling cone 9 and the skimmers 10, 11 are placed at ground potential. A negative voltage of the order of -100 V is applied to the accelerating electrodes 20.
In case a double-focusing mass spectrometer is used as the mass analyzer, an accelerating voltage, i.e., 3,000-5,000 V, is applied to the sampling cone 9 and skimmer 10 and the skimmer 11 is placed at ground potential.
In the structure described above, argon gas is supplied into the plasma torch 3 from the argon gas source 22. The liquid sample 6 is introduced as atomized form into the torch 3 from the nebulizer 21 via an inside pipe 23. Under this condition, when electric power is applied to the RF coil 2, an RF magnetic field is developed, thus producing a high-temperature plasma P. This plasma ionizes sample atoms. The resulting sample ions pass into the interface 8 through the sampling cone 9 and the skimmers 10, 11. The ions inside the interface are converged by the electrodes 19 and directed into the mass analyzer 13.
In the conventional ICP-MS instrument constructed as described above, when a sample is introduced into the plasma, the sample liquid is drawn and atomized by the nebulizer. The sample has been previously dissolved in an appropriate liquid. As an example, where the sample consists of a piece of rock, a semiconductor wafer, or other solid, the sample is dissolved in an acid to prepare a sample liquid. Such sample preparation procedures require a great deal of skill and a lot of time and expense. Furthermore, if the sample content of the liquid is relatively low, then high sensitivity cannot be obtained.
It is an object of the present invention to provide an analytical instrument using a plasma, which directly vaporizes a sample without any chemical or physical pretreatment and can introduce the sample into the plasma efficiently.